Growth hormones the existence of which is common to mammals, avians, reptiles, amphibians and fish are polypeptides that are secreted from the pituitary gland, and there are various reports about growth hormones that were isolated and the structure of which was determined by various research groups. Along with advances in gene engineering, mass production of these growth hormones in microorganisms has become possible and is about to be used for many kinds of livestock. Concerning fish the current situation, however, is that growth hormones are only used on research basis but not applied for practical use.
Since fish growth hormones are responsible for growth promotion of fish, it is believed that if fish growth hormones were available as fodder for hatchery fish, pre-shipment period could be shortened and that this also would be helpful for cost reduction. Natural growth hormones which have been isolated from fish, however, have species specificity to some extent, and therefore, for example, when tuna growth hormone which is commercially available as an agent is given orally to fish other than tuna, although a slight effect may be expected, the effect can not be regarded as sufficient.
Therefore, the inventors of the present application did research for developing a polypeptide which can be used for a variety of fish having universal fish growth hormone-like activity, by exerting supreme growth-promoting activity in fish.